Do you need to let AW know in advance if you plan to attend? - No
Does everyone simply gather and natter about Morgan cars? - Pretty much, or any other random thing you wish to talk about..topics are not restricted, its a casual meet.
What have others who have attended got from going along? - Bit of an odd question, you go because your interested in cars, like the conversation topic being proposed for the talk, same as you would go to any other gathering.
How long are you normally there for (have a busy day so thought a bit of planning makes sense)? - You can leave whenever, usually the talk is 10am and most finish by 11am at the latest, most people leave after.
Do you get trapped in the car park if you want to get away? - If you park in the main carpark as an early arrival you might get stuck for a bit, if you park elsewhere either along the driveway entrance or down the country lane you will be fine.
Edit to add:
https://allonwhite.co.uk/saturday-breakfast-club-events
